This invention relates to apparatus for automatic color balancing of color television signals.
Automatic color balancing has been used to adjust black levels and gains of the red and blue color signals relative to those of the green signals in the signal processing channels following a television camera. Automatic color balancing is particularly advantageous in a telecine camera where the program material has been recorded on motion picture film which may be filmed under widely varying lighting conditions and further subjected to subsequent processing resulting in a wide variation of density and color from scene to scene.
Automatic color balancing, as shown in U.S. Pat. No. 3,786,177, entitled, APPARATUS FOR AUTOMATIC COLOR BALANCING OF TELEVISION CAMERA SIGNALS, has been used to ascertain when substantially black or substantially white signals are to be provided to the colorplexer of a television camera chain. However, there are circumstances during which this circuitry undercorrects, overcorrects or produces a color unbalance. Briefly described, the apparatus of U.S. Pat. No. 3,786,177 compares the amplitude of the red, blue and green color signals against an adjustable threshold level, such as represents black. A gating pulse is generated whenever all three color signals are below this threshold level. The amplitude of the red, blue and green signals at the time of the gating pulse are integrated to develop control signals representative of the average error between the blue and green and red and green signals. These control signals are coupled into a feedback loop to level controlled amplifiers in the red and blue color channels, and balance is achieved when the feedback loop causes the three average color signals to become equal in level. If, for any reason, the relationship between the average of the three color signals below the threshold level does not equal the peak value of each of the color signals below the threshold level, a substantially true signal balance for black will not be achieved. Incorrect black balance, under this type of circuit operation, may be readily understood by the following example in which the scene being observed by the camera contains a background consisting of a deeply folded red curtain. Under illumination, the leading edges of the red curtain folds will appear red in color while the deep recess of the folds will be nearly black due to lack of illumination. A video signal from a scene, such as described, will have a lower than proper ratio of red when the three color signals are averaged resulting in an improper black balance in which the deep recesses of the curtain folds will appear to be of cyan color rather than black.